The present invention relates to facsimile communications over wireless links, and, more particularly, to facsimile (fax) communications over an analog cellular communications channel.
Today, the North American cellular system is predominantly an analog system sometimes referred to as AMPS (Advanced Mobile Phone Service). Although fax communications can take place over an AMPS cellular communications channel, there are some "idiosyncrasies" of fax communications over an AMPS cellular channel that affect the reliability of a fax/cellular connection (hereafter referred to as cellular fax). For instance, a fax receiver provides "delayed feedback" on any error detection and correction to a corresponding fax transmitter. This "delayed feedback" is due, in part, to the fact that fax handshaking and modulation schemes are "half-duplex," and also because of fax error correcting requirements like the "Error Correction Mode" (ECM) as defined in International Telecommunication Union (ITU) fax standards like T.4 (fax encoding standard) and T.30 (fax handshaking procedures). In ECM, transmission of error information from a fax receiver to the corresponding fax transmitter is delayed until after 255 frames of fax information are received. Unfortunately, since there are generally more errors in the cellular communications environment, the effect of this "delayed feedback" is that a cellular fax call generally takes longer to complete than when only land-line connections are involved.
In addition to the above-mentioned "delayed feedback" problem, the cellular fades and hits, which characterize the impairments of the cellular communications channel, can cause a loss of carrier or a large burst of errors. However, a fax modem, generally speaking, is not tolerant to bad line conditions and will tend to immediately disconnect if the received signal quality is not good. Consequently, cellular fades and hits may cause a fax modem to abruptly disconnect.
As a result of the above, it can be observed that the problems to solve in sending fax over cellular communications channels are: enhance the quality of the received fax, i.e., lower the Bit Error Rate (BER), enhance the reliability of the fax transfer, i.e., reduce the number of abrupt disconnects, and enhance the overall speed, or throughput, of the fax transfer.
One approach to improving cellular fax transmission is the use of digital cellular techniques instead of the analog-based AMPS. In the digital approach, a full-duplex digital communications channel is used for the cellular portion of the cellular fax call. In other words, there is no analog modem at the cellular end. Consequently, there are no corresponding fax handshaking and modulation techniques over the cellular portion of the cellular fax call. In this approach, fax-capable terminal equipment at the mobile end of the cellular fax call transmits fax information in digital form over a full-duplex digital cellular channel to a far-end fax-capable modem in a Mobile Switching Center (MSC). The far-end fax-capable modem then establishes the traditional analog-based fax call over the land-line portion of the cellular fax call. The fax-capable terminal equipment at the mobile end communicates directly with the far-end fax-capable modem in the MSC since there is no analog modem at the cellular end.